1. Field of the Invention
The present invention relates in general to a method of producing a body member for a piston for a swash plate type compressor, and more particularly to a method of producing, by die-casting, such a body member having a hollow cylindrical head portion.
2. Discussion of the Related Art
A swash plate type compressor is adapted to compress a gas by a plurality of pistons which are reciprocated by a rotary movement of a swash plate. In general, the piston includes a head portion slidably fitted in a cylinder bore formed in a cylinder block of the compressor, and an engaging portion which slidably engages the swash plate. For reducing the weight of the piston, it has been proposed to form the piston with a hollow cylindrical head section. As one example of the method of producing such a piston, the assignee of the present invention proposed in JP-A-11-152239 a method of producing a blank for the piston, comprising the steps of: preparing a body member including a hollow head section which is closed at one of its opposite ends and is open at the other end, and an engaging section which is formed integrally with the head section; and fixing a closing member prepared separately from the body member, to the body member so as to close the open end of the head section. While the closing member may be produced by any method, the body member is preferably produced by die-casting.
The present invention was made in the light of the background art described above. It is an object of the present invention to provide an improved method of producing, by die-casting, a body member for a swash plate type compressor piston, which body member includes a hollow cylindrical head section closed at one of its opposite ends, and an engaging section integrally formed with the head section.
The object indicated above may be achieved according to any one of the following forms or modes of the present invention, each of which is numbered like the appended claims and depend from the other form or forms, where appropriate, to indicate and clarify possible combinations of technical features of the present invention, for easier understanding of the invention. It is to be understood that the present invention is not limited to the technical features and their combinations described below. It is also to be understood that any technical feature described below in combination with other technical features may be a subject matter of the present invention, independently of those other technical features.
(1) A method of producing a body member of a piston for a swash plate type compressor, the body member including a generally hollow cylindrical head section having a closed end and an open end which is closed by a closing member so as to provide a head portion of the piston, and an engaging section which is formed integrally with a bottom portion of the hollow cylindrical head section which is located at the closed end, the engaging section giving an engaging portion of the piston for engagement with a swash plate of the compressor, comprising the steps of preparing a die-casting device including a casting mold consisting of two mold halves which are spaced apart from each other and butted together in a direction perpendicular to a centerline of the hollow cylindrical head section, the two mold halves having respective molding surfaces; and a slide core which is slidably movable in a direction parallel to the centerline such that the slide core is advanced into and retracted from the casting mold, the slide core cooperating with the molding surfaces of the mold halves to define therebetween a mold cavity when the slide core is advanced into the casting mold, the mold cavity having a configuration following that of the body member which includes the hollow cylindrical head section and the engaging section, at least a front end portion of the slide core having a nonaxisymmetric configuration with respect to a centerline of the slide core; and die-casting the body member using the die-casting device, such that the hollow cylindrical head section has an inner bottom surface having a three-dimensional configuration which is nonaxisymmetric with respect to the centerline of the hollow cylindrical head section corresponding to the nonaxisymmetric configuration of the front end portion of the slide core.
In the method according to the above mode (1) of this invention, when the slide core is advanced into the casting mold consisting of the two mold halves, the slide core cooperates with the molding surfaces of the two mold halves to define therebetween a mold cavity having a configuration following that of the body member which includes the hollow head section and the engaging section. The mold cavity is filled with a molten metal, so that the intended body member is formed in the mold cavity. Thereafter, the slide core is retracted out of the formed hollow cylindrical head section so that the front end portion of the slide core is located outside the casting mold. Subsequently, the two mold halves are moved apart from each other to remove the formed body member therefrom. In the present arrangement wherein at least the front end portion of the slide core has a nonaxisymmetric configuration with respect to its centerline, the formed hollow cylindrical head section has an inner bottom surface which has a three-dimensional configuration which is nonaxisymmetric with respect to its centerline, which configuration corresponds to the nonaxisymmetric configuration of the front end portion of the slide core.
For reducing the weight of the piston, the hollow cylindrical head section of the body member may be subjected to a machining operation such as a cutting operation effected on its inner circumferential surface prior to fixing the closing member to the open end of the head section. When the inner bottom surface of the head section is subjected to the cutting operation concurrently with the cutting operation on the inner circumferential surface of the head section, the inner bottom surface preferably has a configuration defined by a plurality of circles concentric with the head section, that is, coaxial with a cutting tool used in the cutting operation.
However, in order to minimize the weight of the body member wherein the engaging section is integral with the bottom portion of the head section, the inner bottom surface of the head section preferably has a configuration other than that described above.
By effecting another cutting operation on the inner bottom surface of the head section using a drill or an end mill, in addition to the above-described cutting operation on the inner circumferential surface of the head section, the inner bottom surface of the head section can be formed into a three-dimensional configuration which is different from the above-described configuration defined by a plurality of circles concentric with the head section. This additional step, however, inevitably pushes up the cost of manufacture of the piston.
According to the method of the present invention, in contrast, the inner bottom surface of the head section of the body member can be formed into a desired three-dimensional configuration. In other words, the inner bottom surface of the head section may have any configuration, provided that the slide core can be easily retracted from the formed body member. In the present method, the inner bottom surface of the head section has the three-dimensional configuration which is effective to reduce the weight of the body member including the head section and the engaging section formed integrally with the bottom portion of the head section.
The above description is based on an assumption that the inner circumferential surface of the head section of the body member is subjected to a machining operation such as a cutting operation for reducing the weight of the piston. However, the inner surface of the head section, which includes the inner circumferential surface and the inner bottom surface, need not be machined. When the body member whose hollow cylindrical head section has a sufficiently reduced cylindrical wall thickness can be formed by die-casting, the cutting operation on the inner circumferential surface of the head section can be eliminated.
(2) A method according to the above mode (1), the engaging section is a generally U-shaped section having a base section which extends, in a direction substantially parallel to the centerline of the head section, from a predetermined circumferential portion of the bottom portion of the head section, the circumferential portion being offset from the centerline of the head section, and a pair of parallel arm sections which extend from the base section in the direction substantially perpendicular to the centerline of the head section, the slide core being formed with a protrusion which protrudes, in the direction parallel to the centerline of the head section, from a predetermined circumferential portion of the front end of the slide core which corresponds to the base section of the engaging section.
When the U-shaped engaging section is formed integrally with the bottom portion of the head section, a part of the bottom portion of the head section connected to the base section of the engaging section tends to have a large wall thickness. If the slide core has the protrusion according to the above mode (2), a mass of a material which provides the thick-walled part of the bottom portion is depressed toward the base section of the engaging section by the protrusion of the slide core in the die-casting step, to thereby sufficiently reduce the thickness of the thick-walled part of the bottom portion. The protrusion of the slide core is formed to extend in parallel to the centerline of the slide core, so that the slide core can be easily retracted out of the formed body member while avoiding an interference of the protrusion of the slide core with the body member.
(3) A method according to the above mode (1) or (2), the slide core is provided with a squeezing member which is slidably movable in a direction parallel to the centerline of the head section, the step of die-casting the body member comprising forcing an end portion of the squeezing member into a molten metal which fills the mold cavity to give the body member, whereby blow holes present in the molten metal are removed.
The engaging section of the body member tends to have a large wall thickness as compared with that of the head section, and accordingly suffers from blow holes formed therein. In the present arrangement, the squeezing member is forced into the molten metal, whereby the blow holes can be effectively eliminated owing to the pressure applied by the squeezing member.
(4) A method according to the above mode (3), wherein the squeezing member is formed concentrically with the slide core so as to press a central portion of the inner bottom surface of the head section.
With the squeezing member being forced into the central portion of the inner bottom surface of the head section, there is left a hollow residual wall at the central portion. The residual wall can be easily removed by a method according to the following mode (5). Although the residual wall need not be removed since the residual wall is formed within the head section of the body member, it is preferable to remove the residual wall in order to reduce the weight of the piston.
(5) A method according to any one of the above modes (1)-(4), further comprising a step of: subjecting the body member formed by die-casting to a machining operation to cut off a hollow residual wall which is formed at the central portion of the inner bottom surface of the head section, as a result of an operation of the squeezing member, the machining operation comprising rotating a rotary cutting tool and the body member relative to each other about the centerline of the head section.
(6) A method according to any one of the above modes (1)-(5), wherein the step of die-casting the body member comprises die-casting two body members each having the engaging section and the head section, the two body members being connected to each other at their ends on the side of the engaging sections, such that the head sections of the two body members are concentric with each other, and such that each of the head sections of the two body members is open at one of its opposite ends which is remote from the engaging sections which are connected together.
The present arrangement is effective to reduce a cost of die-casting the body member for the piston while facilitating the machining operation to be effected thereon, resulting in a reduced cost of manufacture of the piston.
(7) A method according to any one of the above modes (1)-(6), wherein the step of die-casting the body member is effected according to a pore-free die-casting method.
The pore-free die-casting method prevents a gas from being trapped in a die-cast article, by introducing a molten metal such as a molten aluminum alloy into a mold cavity of a casting mold, with the mold cavity being filled with a reactive gas such as an oxygen, so that the mold cavity is placed in a highly vacuum state owing to a reaction between the molten metal and the reactive gas. The die-cast article formed by the pore-free die-casting method described above exhibits a high degree of mechanical strength with a relatively small wall thickness.
(8) A method according to the above mode (7), wherein the hollow cylindrical head section has a wall thickness of not larger than 1.8 mm.
The pore-free die-casting method described above is advantageous for producing a thin-walled die-cast article. By suitably determining the die-casting condition in producing the body member for the piston, the wall thickness of the head section of the body member can be reduced to not greater than 1.8 mm, 1.5 mm, or 1.2 mm.
(9) A method of producing a piston for a swash plate type compressor having a body member produced by the method according to the above mode (7) or (8), the hollow cylindrical head section of the body member is closed at its one end by the closing member to provide the head portion of the piston, without effecting a machining operation on an inner circumferential surface of the head section.
Since the pore-free die-casting method permits production of a thin-walled die-cast article having high degrees of mechanical strength and dimensional accuracy, the body member formed by the pore-free die-casting method need not be subjected to a machining operation which would be otherwise effected on the inner circumferential surface of the hollow cylindrical head section to reduce its wall thickness. The elimination of the machining operation permits an economical manufacture of the piston. The present arrangement wherein the closing member closes the open end of the hollow cylindrical head section on the side remote from the engaging section assures a higher degree of durability of the piston during use than an arrangement wherein the closing member closes the open end of the hollow cylindrical head section on the side of the engaging section.
While the method according to the present invention is suitable for producing a single-headed piston used in a swash plate type compressor of variable capacity type, the present method is equally applicable for producing a piston used in a swash plate type compressor of fixed capacity type, and a double-headed piston.